In a resin-sealed power semiconductor device (hereinafter just referred to as the semiconductor device), generally, only a front surface of a supporting plate having thereon a semiconductor chip is sealed with a resin. Namely, a rear surface thereof is not sealed with the resin. Therefore, when mounting the semiconductor device on an external radiator, it is necessary to insert an insulation sheet between the rear surface of the semiconductor device and the external radiator, resulting in complicated mounting process.
In order to overcome such inconvenience, a method for sealing the rear surface of the supporting plate is suggested.
As illustrated in FIG. 11, with the resin sealing technique, a semiconductor chip is sealed with a resin using a upper mold 41 and a lower mold 42. More specifically, as illustrated in FIG. 12, a semiconductor chip 45 is electroconductively bonded to a supporting plate 44 as a part of a lead frame 43. The electrodes of the semiconductor chip 45 are connected to external lead terminals 43a with bonding wires 49, respectively.
Then, as illustrated in FIG. 11, the supporting plate 44 is placed in a cavity 46 in the molds 41 and 42, and a sealing resin (for example, epoxy resin or polyimide resin) 47 is injected into the cavity 46. At this time, in order to stop the supporting plate 44 from moving in the cavity 46, the external lead terminals 43a connected to a side edge of the supporting plate 44 and a bar 48 connecting parts of the supporting plate 44 to each other are sandwiched between the molds 41 and 42.
After the sealing resin 47 hardens, the resin-sealed lead frame 43 is removed from the molds 41 and 42 as shown in FIG. 13. Then, by cutting off unnecessary parts of the lead frame 43 such as the bar 48 and a tie bar 43b, a resin-sealed semiconductor device is obtained as shown in FIG. 14.
However, with this method, the external lead terminals 43a of the lead frame 43 are held between the molds 41 and 42 so as to stop the supporting plate 44 from moving when the resin is injected into the cavity 46. After sealing the supporting plate 44 with the resin, the bar 48 protrudes from the resin-sealed section. As described above, after removing the lead frame 43 from the molds 41 and 42, the bar 48 is cut off. Cut sections 50 appear on the front surface of the hardened sealing resin 47.
The cut sections 50 are removed by, for example, chemical etching. However, such an additional etching process increases the manufacturing cost. It is hard to say that performing a desired etching at a rate of mass production is practical. Thus, this method lowers the mass productivity of the semiconductor device.
Alternatively, a method which requires no etching process by supporting and fixing a part of the supporting plate in the molds with a positioning pin is suggested. With this method, an upper mold 51 and a lower mold 52 shown in FIGS. 15(a) and 15(b) are used.
The molds 51 and 52 have movable positioning pins 59 which are allowed to protrude into a cavity 56. A resin filling opening 60 is formed in a part of the contact section of the molds 51 and 52. Unlike the above-mentioned method, with this method, since the lead frame 53 is fixed by the movable positioning pins 59, there is no need to provide a bar on an edge of the lead frame. Thus, every part other than external lead terminals 53a is fully sealed with a resin.
With this method, firstly, as illustrated in FIG. 16, a semiconductor chip 55 is electrically bonded onto a supporting plate 54 as a part of the lead frame 53. The electrodes of the semiconductor chip 55 and external lead terminals 53a are also electrically connected with bonding wires 58. Next, as illustrated in FIGS. 15(a) and 15(b), the lead frame 53 is placed in a cavity 56 between the molds 51 and 52, and the external lead terminals 53a are sandwiched between the molds 51 and 52. An edge of the supporting plate 54 is fixed and supported by the movable positioning pins 59 which are inserted from the molds 51 and 52 into the cavity 56, and a sealing resin 57 is injected into the cavity 56 through a resin filling opening 60.
As illustrated in FIGS. 17(a) and 17(b), before the sealing resin 57 hardens, the movable positioning pins 59 are pulled until their pin points reach the surface of a molding (the inner faces of the molds 51 and 52), and the sealing resin 57 is again injected into the cavity 56 to fill up the portions which were occupied by the movable positioning pins 59. After the sealing resin 57 hardens, the lead frame 53 is removed from the molds 51 and 52. Then, unnecessary parts such as the tie bar 53b are cut off to obtain a semiconductor device shown in FIG. 18.
With this method, by pulling out the movable positioning pins 59 before the sealing resin 57 hardens, the contact section 54a (see FIG. 19) on the front surface of the supporting plate 54, which is in contact with the pin points of the movable positioning pins 59, is covered with the sealing resin 57. Accordingly, it is possible to fully cover the supporting plate 54 with the sealing resin 57, and thereby improving the insulation of the semiconductor device.
However, the pin points of the movable positioning pins 59 used for supporting and fixing the lead frame 53 in the molds 51 and 52 are abraded by a filament contained in the sealing resin 57 (because a large amount of filaments of high thermal conductivity are contained in a sealing resin used for manufacturing a power semiconductor device as the resin is required to have high heat diffusion rate).
If the pin points of the movable positioning pins 59 are abraded and their shapes shown by the broken lines are changed to the ones shown by the solid lines due to the reason mentioned above, the following cases emerge. If the movable positioning pins 59 are lifted to the surface of a molding when the sealing resin 57 hardens to a half degree, the sealing resin 57 flows into a clearance between the upper mold 51 and the movable positioning pins 59 as shown in FIG. 20. As a result, a vertical burrs are formed on the surface of the molding, spoiling the appearance of the molding. Such vertical burrs make it difficult to remove the molding from the molds 51 and 52, and shorten the life of the molds 51 and 52. Therefore, replacing the positioning pins is required as soon as the positioning pins are abraded.
On the other hand, when the movable positioning pins 59 are pulled out after the sealing resin 57 hardens, even if the positioning pins 59 are abraded, it is possible to prevent the flow of the sealing resin 57 from forming the vertical burrs. In this case, however, it is impossible to have an exposed portion preventing effect which is produced by pulling out the movable positioning pins 59 when the sealing resin 57 hardens to a half degree. Consequently, the contact section 54a of the supporting plate 54 where the movable positioning pins 59 are in contact with the supporting plate 54 remains exposed, deteriorating the insulation of the product.